If they could have already produced German quality interiors wouldn't they with Cadillac? Wasn't Cadillac once the industry leader in technology and luxury? They definitely aren't now.
Producing an interior at the level of Audi, Porsche, or BMW isn't as simple as just hiring a designer. The most difficult part is sourcing/ mass-producing the parts and assembling them with more precision than GM has shown they are capable of for quite some time.